This invention relates to shock and vibratory movement control systems and, more particularly, to mechanical snubbers for equipment protection.
Snubbers have long been used on various types of equipment where thermal expansion or vibratory motion can be anticipated and would produce equipment breakdown or failure. In particular, snubbers are employed on such items as piping systems, valves, pump, steam boilers, high temperature and high pressure vessels or chambers. With the increasing use of nuclear power plants, snubbers become increasingly important in order to withstand earthquakes while also allowing piping within the system to expand and contract during its normal operations.
When installed on a particular structure such as a piping system, this snubber typically allows the connected system to expand or contract during its normal heating or cooling cycle. However, when the system is subjected to vibratory or linear acceleration, the snubber controls the responsive acceleration of the system controlling its inertial forces, thereby eliminating any harmful effects.
Many conventional prior art snubbers employ hydraulic systems in order to achieve the desired force damping. Such a system is shown in U.S. Pat. No. 3,376,957 of Alois Baumgartner, issued Apr. 9, 1968. Such hydraulic systems as disclosed in this patent, as well as other prior art hydraulic snubbers all suffer from common problems of leakage, and repeated maintenance requirements and difficulties in functioning through extreme temperature changes and wide radiation levels.
In order to eliminate the inherent disadvantages of hydraulic snubbers, various mechanial snubbers have been developed. The typical prior art mechanical snubbers generally incorporate ballscrews in their construction in order to obtain the necessary conversion from axial to rotational movement. In addition to being extremely expensive, ballscrews rely upon the point contact between the steel ball bearings and the screw itself in order to develop the desired transition from axial to rotary motion. However, under vibratory conditions especially this type of point contact produces brinelling between the ball and the screw flank, thereby eventually causing the snubber to become completely ineffective.
The following patents represent the known prior art which teach the requirement for ballscrews in the particular construction of the mechanical snubbers as disclosed:
Suozzo; U.S. Pat. No. 3,637,176
Suozzo; U.S. Pat. No. 3,669,391
Sasaki; U.S. Pat. No. 3,756,351
Suozzo; U.S. Pat. No. 3,809,186
Yang; U.S. Pat. No. Re. 29,221
Banks; U.S. Pat. No. 4,054,186
These references are typical of prior mechanical snubbers in their incorporation and necessity for a ballscrew assembly in order to achieve the desired conversion from axial to rotary motion. By referring to FIG. 3 of Banks, the typical construction and operation of the ballscrew can best be seen.
Ballscrew nut 27 is threadedly engaged with ballscrew assembly 28 while also engaged via axial slide 26 to end assembly 22. When end assembly 22 moves axially, due to system expansion or vibratory induced movement, ball nut 27 advances axially over ballscrew 28 causing the rotation of ballscrew 28. This induced rotation also causes the rotation of inertial mass 33 which is mounted at the opposed end of ballscrew 28.
This construction is typical of mechanical snubbers and represents the general construction shown in prior art references. Banks also teaches the incorporation of a brake shoe system to control the rotation of the inertial mass.
In Yang, a conventional ballscrew-inertial mass combination is disclosed which additionally incorporates a spring construction and spring clutch arrangement which controllably increases spring tension on the inertial mass during desired conditions. In Suozzo, U.S. Pat. No. 3,809,186, the typical ballscrew system is employed with an additional spring device rotatably mounted along the axis of the screw member in order to control axial advance of the screw member for normal thermal expansions, while also providing stop means for converting the snubber into a rigid strut under shock loading.
In Sasaki, the axial motion is converted to rotational motion through a nut member mounted to a rotational shaft with two inertial masses mounted at both ends of the shaft. Although a ballscrew is not specifically shown, a ballscrew system is described in Sasaki as employable if smoother rotation is desired. Although the ballscrew system is deemed optional by Sasaki, the construction disclosed therein is substantially identical since Sasaki merely teaches an axially moveable nut member mounted to a screw type shaft member for inducing rotation of the shaft member.
In the remaining two references, Suozzo teaches a conventional ballscrew construction in combination with various alternative embodiments of a feedback system which employs the axial movement of the pipe to change the effective length of a substantially rigid strut connecting the moveable pipe to a stationary structure. In both patents, Suozzo teaches the necessity for a rigid strut member connected to the particular pipe in a manner which will allow the strut to move axially in response to various forces imposed upon the pipe. In each instance, the axially moveable strut incorporates a screw member at its distal end which is threadedly engaged with a nut member. The axial movement of the strut causes the nut member to rotate which in turn rotates various combinations of bevel gears to achieve the desired feedback.
Other prior art mechanical snubbers have attempted to eliminate the expense of ballscrews by relying upon frictional means, including the threaded engagement of cooperating members. However, the common problems of heat and wear, associated with friction devices, has shown these types of snubbers to be unreliable.
Two typical prior art snubbers which employ a friction arrangement are found in U.S. Pat. Nos. 3,059,727 of Fuchs and 3,983,965 of Wright. The device disclosed in Wright is capable of allowing a supported element to move relative to a fixed wall below a predetermined threshold, while any greater motion converts the snubber into a fixed, blocked strut. As shown in FIG. 2, this device incorporates a threaded member 69 which is engaged with a second member 49 having internal and external threads and also mounted for rotation. As motion arrester 21 axially moves below the desired threshold, engagement between thread member 69 is fixedly mounted and causes threaded member 49 to rotate about its central axis. In this way, the effective length of device 21 is either shortened or lengthened. However, if a force above the said threshold is transmitted along threaded member 69, shear pin 83 is broken causing threaded member to rotate. This prevents the rotation of threaded member 49 maintaining the strut in a fixed locked condition with the outside teeth of member 49 firmly engaged with teeth 43.
In Fuchs, an energy absorption device is disclosed having similar drawbacks as in Wright. The device in Fuchs employs splined members engaged with cooperating sleeves with ball bearings being employed in order to reduce the effects of friction. Also, friction means are incorporated in the device in order to achieve a threshold with which the devices will not move. When this threshold has been reached, the force will be absorbed by the relative movement of the splined and mating sleeves are converting the axial motion into rotary motion which is absorbed by the friction means.
In both Wright and Fuchs, the device is capable of handling only a single input load in excess of the preset restraint level. Once either system has realized input greater than the preset value, both systems have to be either reset or repaired in order to function again.
Therefore, it is a principal object of the present invention to provide a mechanical snubber which is inexpensive to manufacture and provides repeated and reliable performance.
Another object of the present invention is to provide a mechanical snubber having the characteristic features defined above which eliminates the necessity for ballscrews.
Another object of the present invention is to provide a mechanical snubber having the characteristic features described above which substantially eliminates binding of the operating components.
Another object of the present invention is to provide a mechanical snubber having the characteristic features defined above employing radial line contact of elements in order to convert the axial motion into rotary motion, thereby eliminating potential brinelling of the cooperating elements.
Another object of the present invention is to provide a mechanical snubber having the characteristic features defined above which is lightweight, shorter, and completely interchangeable with speed and accuracy.
A further object of the present invention is to provide a mechanical snubber having the characteristic features defined above which eliminates any requirement for structural welds as well as eliminating the necessity for critical tolerances between moving parts.
Other and more specific objects will in part be obvious and will in part appear hereinafter.